27 lines
567 B
C++
27 lines
567 B
C++
#include <bits/stdc++.h>
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using namespace std;
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//判断是不是质数
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bool isPrime(int n) { //最快的方法
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if (n < 2) return false;
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if (n == 2 || n == 3) return true;
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if (n % 6 != 1 && n % 6 != 5) return false;
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for (int i = 5; i <= floor(sqrt(n)); i += 6)
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if (n % i == 0 || n % (i + 2) == 0)
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return false;
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return true;
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}
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int main() {
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int n;
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cin >> n;
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int count = 0;
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for (int i = 2; i <= n; ++i) {
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if (isPrime(i))count++;
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}
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cout << count << endl;
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return 0;
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}
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